Recently, electronic devices are rapidly becoming portable and cordless. Such portable electronic devices usually include secondary batteries as the power source for driving the devices. There is thus demand for development of secondary batteries which are small and light-weight and have high energy density.
However, in order for secondary batteries to be used as the power source for driving portable and other devices, the secondary batteries need to be charged. While they are being charged, the devices cannot be carried for use. As such, fuel cells are attracting attention because they can be continuously used as the power source for a long time if only they get refueled.
Fuel cells typically have a stack of unit cells (cell stack). Each unit cell includes a membrane electrode assembly (MEA) comprising an anode, a cathode, and an electrolyte membrane interposed therebetween. The cell stack is composed of a plurality of MEAs stacked with conductive separators interposed therebetween. Both ends of the cell stack in the stacking direction are fitted with end plates. A fuel is supplied to the anode of each unit cell, while an oxidant is supplied to the cathode.
Among fuel cells, direct methanol fuel cells (DMFCs) in particular are under active development. DMFCs use methanol, which is in liquid form at room temperature, as the fuel. Therefore, compared with fuel cells that use, for example, hydrogen, which is in gaseous form at room temperature and normal pressure, as the fuel, DMFCs can easily reduce the size and weight of their fuel supply system. Therefore, by using a DMFC as a power source, it is possible to realize a portable device with good transportability. Also, the fuel can be easily resupplied.
However, DMFCs require time to stabilize the oxidation reduction reaction, which makes it difficult to promptly adjust the amount of power generation in response to load variations. Therefore, in a power supply apparatus using a DMFC, it is desirable to respond to load variations by operating the fuel cell at the point at which the largest power can be generated and storing surplus electrical output in a storage battery.
From such a viewpoint, a fuel cell system has been proposed in which the voltage and current of a fuel cell are constantly measured and the fuel cell is operated at the output voltage and output current at which the largest output can be obtained (e.g., see PTL 1).